Veterans with posttraumatic stress disorder (PTSD) have poor physical functioning and there are no existing interventions that effectively address this problem. Our long-term goal is to develop and implement effective interventions to improve physical functioning and reduce risk for adverse health outcomes for Veterans with PTSD. The overall objectives of the proposed project are to develop and pilot test a novel intervention aimed at initiating and maintaining higher levels of physical activity for Veterans with PTSD. This new intervention will address attitudinal and behavioral barriers to increasing physical activity for Veterans with PTSD. Particularly relevant barriers for those with PTSD may include low perceived behavioral control for making positive long- term lifestyle changes and personal attitudes about the harms (vs. benefits) of physical activity. The proposed work will meet two specific aims: 1) Adapt graded exercise therapy (GET) and incorporate motivational interviewing and mobile health technology (mHealth) to increase physical activity for Veterans with PTSD; and 2) conduct a pilot study to examine the feasibility and acceptability of the intervention for a future randomized controlled trial. To achieve Aim 1, we will develop treatment manuals, training procedures for interventionists, and processes for assessment of intervention fidelity. The intervention will use GET to increase perceived behavioral control by enhancing knowledge about appropriate activities and helping participants set appropriate and attainable daily goals for physical activity. We will also incorporate motivational interviewing techniques and mHealth to enhance positive personal attitudes about physical activity. We will work with our Veteran Engagement Panel for PTSD to refine topics and communication materials. In Aim 2, we will enroll 3 groups of 6-8 participants (18-24 total) and use a concurrent mixed-methods approach to rapidly assess feasibility and acceptability. Quantitative measures will include recruitment (proportion enrolled out of total eligible); attendance (proportion who attend 75% of in-person visits); and retention (proportion who complete post-intervention data collection). Qualitative assessment will consist of semi-structured interviews on acceptability of intervention components (eg, format and topics of in-person visits, usability of mHealth), and barriers and facilitators to attendance. At baseline, during, and post-intervention, we will also measure overall and physical functioning, and collect data on other clinical variables (PTSD symptoms, depressive symptoms, pain, fatigue, and sleep quality) to inform a future effectiveness trial. The proposed work is innovative because it is a novel combination of GET, motivational interviewing, and mHealth to increase physical activity. It will also be the first intervention to focus on changing physical activity as a means to improving physical functioning for individuals with PTSD. Successful experiences with this program may also help participants build confidence about their general ability to make positive changes related to health, thus increasing the likelihood they will engage in (and adhere to) mental health treatments. If such synergy is ultimately demonstrated, it will open a new direction for VA RR&D and VHA clinical care by indicating that interventions focusing on physical functioning should be more closely integrated with mental health services. Additionally, this project will be one of the first to collaborate with our Veteran Engagement Panel on PTSD research. The format of our panel is adapted from and builds upon previous models of patient stakeholder engagement. Our model addresses many of the barriers to effective stakeholder engagement, including challenges to timely recruitment of patients with relevant experiences, and lack of facilitation expertise among investigators. Completion of this project will help establish the utility of our model of Veteran stakeholder engagement for rapidly developing effective interventions with improved feasibility and acceptability. Thus, this work will enable incorporation of Veteran engagement in future VA research projects.